Did you turn the heater controlls inside the car to full heat when you turned the heater on? The heater controll valves MUST be fully open, or else they will block any effective circulation.

You can check if they installed it in the right place by following the heater hoses. The heater should be installed in the heater return hose from the firewall to the waterpump. On my W115, this hose runs along the passengers side of the engine. There might be an arrow on the side of the heater as well. This should probably point towards the water pump (where the hose connects with the engine).

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1971 220D/4-speed ~250K mi. Family owned (dads side) through three generations since <5K miles.
1992 Chrysler LeBaron, 3.0V6 125K. Family owned (moms side) through three generations since new.
1977 Chevy Camaro Continual hotrod project
1970 Cadillac Sedan DeVille 472ci and nearly 19' long
1974 Fiat Spider Still needs work