Turning on the heater makes me lose engine power?

The only thing I can think of is some kind of obstruction in the heater core and/or hoses.

Does the problem happen whenever you put the fan on, or only when it's up to temperature? If it's all the time then you likely have either an earth problem to the heater blower motor or a knackered battery or alternator. A garage can check the battery and alternator easily enough with a volt meter. If the battery is less than 10v when the engine is off, then the battery is likely the problem, if it's the alternator it will be less than about 13.5 v- 14 v with the engine running. If either is the case you should have the same problem when you switch all the lights on and/or the rear window demister too. (If it's the battery the car will struggle to start - with a half decent battery my 190E's always started straight away). However, the radiator fan on the 190E is driven by a viscous clutch (powered by the engine) rather than an electric fan and therefore loads the engine when it cuts in. If the clutch is knackered, this may cause you problems. However, the fan turns on once the engine temp is up to about 90-100 degress C on the temp gauge. If the problem happens when the radiator rather than heater fan kicks in then the clutch is likely knakcered or the belt could be too tight, causing too much load on the engine and a loss in power. The clutches go after a while (most 190's have had that problem already, if yours is low (below 100K) mileage it might be on it's way out. I'd suspect the battery first, then the alternator, then the clutch..