This one lists HUMMER in the "it fits" list (yours is a 3157 bulb I believe)
http://www.autobarn.net/des20100.html
Back-up Alert Bulb #1156 fits the following vehicles:
Acura: All but CL 01; MDX 01; RL 96+; TL 95+; Alfa Romeo: All; AM General:
All Hummer; Audi: All; BMW: All; Buick: Century 90-92, 94+; All Electra; All Estate; LeSabre 90-96 etc etc etc ....