possibly relay, thats what controls the "high" speed. otherwise theres a connection thats known for this problem in late 70s to 90s gm vehicles. found this...
Start at the blower motor. Its wires will be purple and black. The blcak one goes to the chassis; the purple one goes to the relay, which has a plug with 5 wires in it. One of those wires is a thick red one. Follow that wire across the engine bay, under the windshield cowl; about at the right valve cover, you will come to a connector, of monumentally STUPID design. Its meltable plastic housing is actually responsible for holding the tension on the 2 metal parts, which gives an absolutely ironclad unstoppable GUARANTEE that it will fail. You will find that it is burnt to ashes.
Cut the terminals off of both of the thick red wires joined by the above connector. Throw them, and the remains of the connector itelf, into the nearest govt-approved toxic waste incinerator for proper disposal. Replace them with the BIG WIDE THICK slide terminals. Not, the little weenie ones that are just like the ones with the small red ends; the BIG THICK yellow ones. They're about half again as wide and as thick as the standard ¼" ones. Fully insulated is best; but if you can't find them like that, use some sleeving, heat-shrink, a thick wrap of tape, or something, to keep this connection which has hard battery on it, from touching something and starting a fire.
Then enjoy your high speed blower again.
Next, remove your HVAC control head from the dash. Find th e"mode" switch, near its center. It will have a number of wires that go to the fan speed switch and elsewhere. One of those wires will be brown. Follow it about 12" back into the dash, to where it plugs into the brown wire that supplies 12V to the entire HVAC system. You will observe that it is the same idiotic design from the pen of the same moron. Replace it as well, so that next week, you're not forced to post something about "my whole HVAC system stopped working".
