Heater in '89 560SL

I recently bought 560SL. Heater takes up to 30 minutes to heat up interior at 40*F outside temp. Temp gauge registers 85*C (185*F). Even when temp wheel is set on "Max", air blowing from vents is cool, maybe 75*F.

It acts like thermostat is stuck in open position. But gauge shows engine reaching normal operating temp after a couple of miles of normal driving.

Af first I thought this may just be the way MB climate control works. Thought I would seek some help for DIY repair.