According to the 1978 service manual the battery terminal had a purple wire coming from it that connected to an 18 gauge fusible link wire and the other end of the fusible link connected to your purple/white wire. You can get the fusible link wire from any good parts store, they just need to know what gauge the fusible link wire is.
Hope this helps,
If you’re stuck on a tough Mopar performance or restoration problem we have someone who may be able to help with fresh information and resources about classic Dodge and Plymouth parts. Matt Deane has been involved with Mopars all his life and is the Mopar Performance Parts specialist at SoCal’s Glendora Dodge. Now he’s offering his wealth of knowledge to Mopar Max readers and will track down the answers to your tough questions. Just fill out the form below and we’ll include his answers every month right here in Mopar Max.