OMG how daft.

[TVRleigh]

Just found out what my balast resisters are being used for on my car.
as the wires drop down and got a bit melted on the exhorst.

Followed then all the way back, and they are both connected to the internal dash lights for when you have them on dim.

2 questions
Why have them in the engine bay
and why not just use a varible resister pot, and just fit a knob on it.

Im going to remove and just fit a varible one. unless anyone know's why I should not.

[adam quantrill]

Probably because it'd have to be a hefty pot, probably dissipate 10W or more.

As you seem OK with soldering why not put together an electronic 'dimmer'. The theory is simple, put together a bistable with one side controlled by a normal pot. Get one of the transistors to drive a power transistor, which feeds the dash lights. As you vary the control pot, you vary the mark/space ratio, and go from dim to bright.

I tried something like this as a train controller, and it worked, but the problem was the train engine also acted like a loudspeaker and made a teriffic whining noise.

[TVRleigh(BBWR)]

May just goto the breakers are take one of a newer car.

[Wedg1e]

On the early Tasmins the dimmer resistors were just several yards of instrument wire (not even proper resistance wire), coiled up and taped behind the dashboard. The heater fan 'slow' speed was created using the same principle.
The SD1 column has a rheostat built-in, but extended use can burn that out as well.

[leorest]

My instrument lights offer two distinct light levels. Too dim, and off.

The rheostat is always set to the "Too dim" position.

[TVRleigh(BBWR)]

You may just have one of the wires come off.
Does your knob have 3 positions.