I have a 70 Challenger that has a fusible link connected to the red wire under the dash and don't see where it would connect. It has the same lug and connector on it as the one on the starter relay. Not sure if this is supposed to be there as the wiring diagram in the service does not show it. Any help would be appreciated.

Sounds like it was added. On my '70 the fusible link was under hood. In general, the link protects from overloads/shorts from the main power sources: battery and alternator. Both are under hood so I wouldn't see benefit from a link in the cab.

Perhaps there was a mishap in the past... especially good idea to check main power connections at the bulkhead, ammeter (if equipped) and in cab column connectors.

Also worth mentioning the 1/4" blade connector is a weak spot on OE links. On my re-pop harness, I replaced the blade with two eyes and a bolt + tape after a connection failure.

This would be correct for a 70 Challenger:

http://www.linric.com/challenger/Assembly2-1.jpg

Yes that is how my engine compartment is wired. I'm trying to insert a photo but am not having any luck.

There was no factory fusible link under the dash on a 70 Challenger. The fusible link coming off your starter relay goes to the ammeter.
Who knows why someone added one at your dash............