W126, Accessories

1982 300sd - heater blows warm for about 5 minutes then heated air turns cool. should i replace monovalve, then try solenoid that activates. turning heater to off then back on sometimes results in heat flowing again, but not always. turning car off then restarting returns heated air. i think malfunction in monovalve, but solenoid may be at fault as well. how do you diagnose which piece to replace or both? any sources for solenoid besides dealer?? i noticed you are sold out of solenoid

I just replaced my differential mount. Yes, it fixed the rear sag problem!! One of the 4 13mm bolts that hold it into position on the subframe snapped off inside the subframe while removing the old mount. Is there a way to get that piece out of the hollow subframe member so I can replace the bolt (assuming the threaded insert is ok...I suspect it may be rusted, thus the break)? Are bolts 3 enough to hold the mount in place?