The song Don't Whiz On The Electric Fence is indeed from the cartoon Ren and Stimpy.
The complete story behind the song is that it is from the commercial for Ren and Stimpy's favorite boardgame.

America's favorite idiot Beavis and Butthead also had a song about Whizzing on an electric fence:

When I was little and I had no sense,
I took a whiz against the electric fence.

Hurt so bad, shocked my balls.
Took a crap in my overalls.