1989 Mercury Station Wagon Heating Problem

I recently purchased a 1989 Mercury Grand Marquis Colony Park Wagon. Everything seems to be working fine except the heating system. This has the Auto Climate Control. Car does just fine on the cooling side. However, when I want heat this is what happens. When I start up the engine from a cold start and warm it up it seems to heat for a while. Then it will start to get cold and stays cold or luke warm. Please advise. Thanks!