1983 240D block heater

[Printprocc]

I can't for the life of me, find where the @#&? to plug in the block heater. I bought my car six months ago when it was warm so it didn't matter, but I guess I am a grasshopper about hooking this thing up.
I have had three different opinions as to where this little heater is.
My local dealership doesn't service this model anymore (too old) who are they kidding.
Please can anyone out there help me find this little bugger?

Thanks Paul