87 5kcst electrical/climate control help needed !!!!!!!!!!

[Leon]

Do the license plate lights work? Its all on the same fused circuit. I
assume you already checked that, but you may have missed it. Make sure
that all other lights in and outside the car work, as some of them are
connected.
I never heard of a instrument light booster, just had a look in ETKA
and couldn't really find anything. I have no clue how it looks like
etc, or how it would work?! How is the voltage regulator? did that
check out?
You might be looking at the hard to trace intermittent connections or
bad grounds.

Good luck