FIX. Just installed GM p/n 88973260 (ACDelco 15-80202). Spent approx. $40. Now have distinct and satisfactory heater flow for P2, P3 and Hi. The old p/n 15359315 did not fail to Hi only, but ran on P3 also.

Is it just me or do any of you find P2 flows too little to keep you comfortable and P3 flows too much (too hot, too cold, noisy) ? I've just come from a car (Saab) that had 10 blower speeds, so I'm being picky. If the resistor card were old-school, I would probably modify the resistance value on P2 ...