It would depend on which defrost system is not working, the rearview mirrors, back window or front window.
check the grid in the back glass to make sure there are no nicks in it
Back in 1970 all engine controls were done by mechanical means. The answer is no, there is no PCM on a 1970 VW Beetle.
Its for your back window defrost
VW Beetle
yes. but you will have to yous the old seal from the 1969
Model year 1970.
1500 dollars
165R15 is stock.
There is a switch on the right side of the instrument cluster near the hazard (4-way) switch. If that doesn't work check the fuse. If that is not the problem check the connectors at the rear window. There are testing procedures for testing the heating elements, they may not be working.
One is the Heat on/off and the other is the Defrost/Floor lever.
Why the defrost may not work in a 1993 Chevy Caprice Classic could be because the heating core is out.