Classis Industries might be "one stop shopping" for heater case kit, body plug kits, wiper pivot seal kits, everything needed. If we assume they acquire the items from various producers like DMT, etc.., we might expect a retail price difference.
I’d never buy anything from Classic I could get somewhere else. Their customer service is lousy and their prices aren’t that great.
DMT is the manufacturer of the heater box rebuild kits, they make all the gaskets and seals in house and everything you need is in the kit. They have great customer service, there’s no reason not to buy straight from DMT.
The DMT kits do come with instructions that have pictures showing all the major components of the box. The picture quality isn’t perfect, but it was good enough for me to walk me through restoring my own heater box. Your other option is a service manual for you car, between the two you should be able to figure it out.
