I posted this on the w-body board, figured someone might find this useful here. The 1990 speedometer cluster pinout (should be the same as 89):
C1 - not used
C2 - not used
C3 - oil gauge input (tan)
C4 - coolant temp gauge input (dk grn/wht)
C5 - ign from INDIC fuse (pnk/blk)
C6 - not used
C7 - not used
C8 - ground to fuel gauge sender (blk/wht)
C9 - charge indicator (brn)
C10 - left turn (lt blue)
C11 - hi beams (lt grn)
C12 - right turn (dk blue)
C13 - fasten belts (yel)
C14 - not used
C15 - SES light (brn/wht)
C16 - not used
D1 - not used
D2 - not used
D3 - fuel gauge (ppl)
D4 - vehicle speed input (dk grn)
D5 - ign from INDIC fuse(pnk/blk)
D6 - shift indicator (tan/blk)
D7 - tach input (wht)
D8 - ground to G103 (blk)
D9 - illumination (gry)
D10 - ign from cluster fuse (pnk/blk)
D11 - not used
D12 - not used
D13 - not used
D14 - brake indicator (tan/wht)
D15 - anti-lock indicator (gry/wht)
D16 - low coolant indicator (yel/blk)