It won't I'm afraid.
Depending on the age of the vehicle, you can "upgrade" your heater by fitting one from a later vehicle.
The earlier vehicles had a heater with pipes that turn downwards out of the side of the box. If you can find a heater with a pair of plastic pipes which come straight out, (from a 300 series onward I think) then you should find they are a bit more efficient.
The other alternative is to fit a V8 and an auto box... no shortage of heat there !!