Usually having both the Seatbelt and Airbag (INFL REST) light on, especially in a 1990 MY, means the Central Control Module (CCM) has detected a poor ground condition and stored a code (16 or 26) which is most likely due to a "Poor Sensor Ground" on one of the 2 Supplamental Inflatable Restraint(SIR) sensors located on the front frame rails.
Don't spend his hard earned money soooo fast. You can turn the light off by following the sequence set forth by G.M. by turning the key on and off, grounding and ungrounded the appropriate diagnostic leads for the 3 second intervals.
Okay, you might have to do it two or three times, but you get the hang of it pretty quick--for free (less a piece of wire or paper clip)
1. Turn ignition off
2. Ground ALDL Terminal K (this is the lower row, 3rd from the right).
3. Turn ignition on
4. Wait for fault codes to begin flashing (approx 5 seconds). When the codes begin flashing, unground ALDL terminal K for 3 seconds + or - 1/2 second.
5. After ungrounding ALDL K as described in step 3 above, ground it again for 3 seconds + or - 1/2 second.
6. Unground ALDL K and wait 10 seconds. Before turning the ignition OFF, verify that code 12 is the only code being flased (this is the seatbelt/air bag light flashing). If not, codes have not been properly cleared. Repeat from step one if codes not cleared until the codes clear.
When I do this, I put a jumper wire on K and used some aligator clips to ground it to the driver door hing (cuz I have the door open the whole time). You can ground, unground th aligator clip to the jumper to clear the code.