From the _95_ FSM! If GM changed something between years, this might not help. But if it is the same, you should be good.
Switch in "park"
Probe I/P lamp dimmer switch with a test light between term 'd' (yellow) and ground.
Is light on?
--No: Check for poor connection at C200 term 'B8' (yellow).
------Check for open in ckt 32 (yellow) between I/P lamp dimmer switch term 'd' and I/P fuse block.
------Check for open in ckt 9 (brown) between I/P fuse block and headlamp switch.
--Yes: I/P dimmer switch to 'max'
-------Probe I/P dimmer switch with a test light between term 'a' (dk green) and ground, is test light on?
--No: Check for poor connection at I/P lamp dimmer switch connector. If OK replace switch.
--Yes: Probe I/P dimmer switch with a test light between term 'a' (dk green) and term 'b' (black), is test light on?
--No: Check for poor connection at G200.
------If G200 is clean and tight, repair open in ckt 450 (black) between G200 and S201.
--Yes: Probe I/P lamps dimmer with a test light between term term 'b' (dk green) and ground and term 'd' (yellow) and ground, is test light on for both terminals?
--No: Repair open in ckt 32 (yellow) or ckt 44 (dk green)
--Yes: Probe I/P lamps dimmer with a test light between term 'c' (gray) and ground, is test light on?
--No: Check connectors at I/P lamps dimmer connector. If OK replace I/P lamps dimmer.
--Yes: repair open in ckt 8 (gray) between S209 and I/P lamps dimmer term 'c' (gray)
You already seem to know where the dimmer switch is.
The dimmer though is on the side of the steering column, connector has three wires. This is the thing that makes the speedo go dim when the lights are on.
C200 is the 56 cavity Body to I/P harness, it's bolted above the accelerator pedal.
G200 is a ground located at the base of thge LH A pillar behind the LH kick pad.
S201 is a splice located in the I/P harness 8cm from the C200 break out.
S2090 is a splice located in the I/P harness 20cm from the Ambient Light Sensor Connector break out.