you can fit an auxillary heater really easily, that will work well assuming your engine is reaching a decent temp. Do you have some sort of a
Winter Front on? You'll never get any decent heat if your engine doesn't get up to a good operating temp.
some people have used the
Mojave heater as an aux and been pleased with it, or you could get a rear heater from a TLC.
Another fellow I know fitted one of
these heaters and says it easily roasts him in his 109.
Make sure your door seals are good, your bulkhead doesn't have holes etc, they all add up a lot.