There is a resistor setup. It's in the fan housing, and accessible (with small hands) if you remove the passenger air vent & lower (black plastic) dash panel.
However, it's normally the fan switch itself that loses contact internally (I've never had resistors go, but I've done about 10 switches).

The heater has a blend door, which is linked to a heater tap (which is part of the plastic pipe that exits the firewall). There are 2 heater pipes out of the firewall, one is brass, the other plastic.
These are fully mechanical, ans rarely give trouble (normally the heater valve rusts away & you can't stop the heat). You can check the cable - it's like a choke cable, running down the passenger side of the dash, connecting to the heater box below & behind the switch panel. It has a wire loop that hooks over a lever, and the outer sheath is simply pressed into a retaining clip that provides the adjustment to align the scale on the switch.

If that cable is connected, and there's resistance on the hot/cold slider, then I'd be saying your problem is more likely a corroded (& blocked) heater hose fitting on the water pump or thermostat housing, or a thermostat that isn't working (or not there) and thus not restricting the flow & allowing pressure to feed the heater core.

Hi John,

Mirrors:
VK & VL use a pair of 5-pin changeover relays (ie one with 87 & 87a terminals, not just 2x #87 terminals).
Use VL diagrams to work them out, as the VK factory manual has an error in it. I do have the complete VK setup (incl locks & windows) so I can send you pictures of what wires go where on the relays if you really get keen and want to make your own harness, but there is another option:

I find it easier to use the switch from a VS or VT-VZ, as they don't run the relays - you only need power, earth, and 3 wires out to each mirror. If you're not worried about originality, it's a better method, and they are far more readily available.

Air-con
I've done quite a lot on the aircons on VK & VL (including 3 ground-up rebuilds).
If you haven't had gas in the system for some time (the fact it won't click in suggests so) then it's unlikely you'll get it to work reliably unless you renew the entire gas side of things.
They corrode out internally in the pipework & cores, and the white powder travels through the system, destroying everything in it's path (the filter/dryer is after the condensor, so corrosion in the evap core goes straight into the compressor).

Apart from the hard lines, all new parts are available that can work (TRS090 compressor needs a little work on the mounts, and a bigger belt, but it's a brand new replacement for the no-longer-available TR70).

You can't get a VK evap core, but you can get the later VL type (used from late 87) which needs a 1988 VL evap box to fit it (trust me, it won't fit in the earlier VL or VK box - I have tried, and that's how I learnt about it - the aftermarket catalogues are incorrect...), but once you have the new core & TX valve in the later VL box, this section does bolt straight into the VK's system.

New heater cores are available, but the pipes are way off, so I solder the pipes from the old core into the new one. VK & VL are different in the shape of the heater core pipes too - they exit the firewall on different angles - another reason to re-use your old core's pipes that fit your car.

You'll also need new flex hoses, as the old ones don't work properly with R134a (and most "retro-fits" never changed them) - take your old ones in and they can redo them with new rubber hose that is compatible with the R134a gas.

TVS - there's 3 systems operating.

If you study the emission controls vacuum diaphragms, you'll find that these don't really hinder performance that much when it's all working well. The problem a lot of people have, is they simply block every hose - if you block the one going to the EFE valve (actuates the butterfly in the exhaust) it will make it run a lot worse, but you can't tell those people, and subsequently all the pub stories start circulating. These days most of the talk about early 80s commodore emission control seems to have been replaced with "cop chips" that give V8 supercar performance from a V6.....

As you have EST, the timing is controlled by the computer, not vacuum advance, and it reads from a MAP sensor fitted under the fuse box, so there's not a lot you can do about it. Most inspectors & highway patrol cops these days don't really understand the old systems (a lot of them weren't even born then) so the chances of having issues with the law are pretty slim unless you're a young hoon hanging around in the wrong places or driving around with "pick me, pick me" foglights on.

Personally, I'd re-route the hoses so you open the EFE valve (triggered by the green sensor in the head) after the car has warmed up (it does help the warm up whilst on the choke) - assuming you still have the original exhaust manifold on it. If the EFE actuator isn't free & working easily, or the thermal switch is stuffed, then remove the blade from the butterfly valve in the exhaust at the manifold join and put the empty housing back in as a spacer - the car will just take longer to warm up.

As far as spark control on a non-EST - done by the black sensor in the thermostat housing, and the solenoid mounted nearby, I simply re-route it so it gets vac advance in all gears, not just top. There's also a confusing array of hoses that feed full vacuum to the dizzy in overheat conditions, as advanced timing at lower RPM (and no load) helps cool the engine.
The last one is the EGR (brown sensor in the head) - this is one you can safely plug without harming performance.


Gaskets:
I haven't had a Holden 6 for over 15 years now, so I wouldn't know where to start, but you used to be able to buy a black neoprene rocker gasket from almost any parts store. It had a thin steel shim inside it to retain the shape. I found it great as it was re-usable, which was very convenient when adjusting rockers all the time (I also put short studs into the head as well - makes the rocker cover a 2-minute job, as you're not having to fight each bolt.

Phew, I think that covers it all?

If you have full speed on the fan, and you've cleaned the small fan switch, then it will be the resistor pack at fault (see further up for that info).
I usually give the copper circuit board for the "mode" switch a light clean with emery paper (240-400 - whatever is laying around) and a smear of new electrical contact grease. Same with the fan switch.

If you're fitting a HEI dizzy, run vacuum directly to it - that will bypass the spark control TVS.

These little niggles were always around in these cars, even back in the 90s when they were half the age they are now - it's just a fact of life, and you'd get the same with anything that age. The VK has a few more complex things than say a HQ-HZ, but thankfully nowhere near as many as later cars. As you say, just do one thing at a time & get it sorted step by step.