Hey 142 guy, I'm having this same problem with the dash light rheostat.
You say it can be set to a fixed illumination level with the addition of a resistor.
What resistance does it need to be? And should it be on
I would be inclined to go for the robust solution with a complete by-pass of the rheostat with an external shunt). This will give you full brightness. If you dislike full brightness everything gets more complicated. Volvo does not provide a test value for the rheostat so you would have to try and measure the maximum resistance of the rheostat. This value would represent full dim. You can pick anything between 0 ohms (shunt) and the full value of the rheostat. The complicated part is that if you decide to use a resistor in the shunt, it needs to be sized correctly for the power dissipation. To calculate this, you need to measure the current consumed by the 5 interior lights with full 12 volts applied to them. Lets say that this turns out to be 0.5 amps. The power dissipated in a resistive element is given by the formula (I^^2)R where I is the current and R is the resistance. If you were using a 5 ohm resistor with that 0.5 amps the power dissipated in the resistor would be 1.25 watts so you would typically select a 2 watt resistor (1 watt and 2 watts are common power ratings).
The resistance values you will be looking for will likely be in the low numbers (less than 10 ohms) and not common. You may need to go to an on-line vendor like Digikey to get the selection of resistance and power rating you need if you want less than full brightness.
As I noted, full bright is much more straightforward.