A/C to Heat ONLY conversion on 68 Dart Sports Special

Another way is to buy an aftermarket heater. I saw one the other day very small, for about 200 new Look ebay, too.

Here's one, very small

https://www.summitracing.com/parts/...-flex-a-lite&gclid=CIbV2sbortMCFUGTfgodkJoD9g