That indicates a really poor connection that is passing minimal current to run the dome light, and when you apply other loads, it dies. Check over the bulkhead connector, check the ammeter for loose connections. Check over connections on the starter relay which are often used for a junction point.
I don't know exactly where the fuse link is located in those old girls, maybe you got it caught on something and tore it loose when pulling/ installing engine parts. Or it may have corroded and losing connection.
