[rcweingart]

Okay, first I replaced the factory heater valve with a simple in-line heater valve from NAPA but any in-line valve without a bypass line will work. That goes on whichever of the two lines into the heater core that the original valve was on. Next, on the other line (and there are only two lines into the core, one supply, the other return - and it doesn't much matter which) install a simple in-line shut off valve. The trick is to completely isolate the heater from any by-pass or leakage. The reason most SPF coupe ACs don't work well is that hot water leaks through to the heater core, through which the AC air also passes, and heats up the core and so the AC air as well. Isolating the heater core from engine hot water reduces the heat in the heater core and the AC air stays cold. In cold weather when you're not using the AC, turn the shut-off valve back on so hot water can get to the heater core and you've got heat.