Check the infamous ground in the blower motor conenctor behind the glove box first. When mine was messing up it would cause it to go to the floor and the air to get warm as well as going to outside air. Fixed teh ground and no more issues. Control heads will also do this though. If it is for the 99 in your sig, there is a DIY here on how to repair the 96-99 control heads with a ad on relay.
Mine was doing this too. R134 was low, vac'd recharge, re-did it a couple times with slightly diff lbs each time but never got it to do real well. High side is 'bouncing' a little bit and now neither system is cooling more than 20 degrees below outside (slightly better than before). Before; front was cooling 15 below outside and rear was probably 20-25. I'm going to change my orifice tube since it has a new compressor from the PO and it's cheap. It would be nice it that fixes it but my hopes are low.
I was thinking that I may put a valve in the heater hose to make sure I wasn't reheating my air in front with the door not moving completely. I am sure someone has better input than me but maybe we can help each other. I will let you know what happens.