You can pull it out without unbolting anything. If you have small hands (or a helper with small hands, I guess) you can easily reach behind the instrument panel and disconnect everything, then it'll be free. The only thing to watch out for is the oil pressure gauge: mine has a oil line that runs from the oil filter housing to the back of the instrument panel that has to be unscrewed, but yours (84 300D turbo) might have the electrical sender. I don't know.

I replaced my rheostat with one I pulled from a junker. The old one had the same symptoms as you describe, the "new" one works just fine and cost 25 cents at the local U-Pull-It junkyard. Others recommend bypassing the rheostat altogether by jumping it with a piece of wire.

- Nathan
'83 240D "Steiner"
'00 New Beetle TDI