I have an 02 e350 with the 7.3

I have the factory rear heat/ac unit but I'm getting nothing on the lower settings. Only when I turn it to high do I get any air flow out of it. I have checked all the fuses and replaced the resistor pack for the rear blower motor already and that didn't solve the issue.

I have purchased the shop manuals, but they haven't arrived yet so I'm wondering if anyone knows where I can start tracking this down.

The other question I have is where do the rear controls usually plug into the wiring harness? My dash controls have a setting for "rear control" but I have no rear controls in the van. It was previously a cargo van so I'm assuming the previous owner removed the controller and used the controls from the dash only.

I don't see an obvious spot on the wiring harness for a rear controller to plug into...

If you don't have the Club Wagon style full headliner in the rear area its odd you'd have the dash control with "rear control" as an option for that blower selection. That's somewhat odd but not the source of your issue. If your van has always been a cargo-only vehicle provisions for the rear heater controls most likely don't exist or weren't removed---just never installed at the factory. You might say the current control pod is "wrong" for the chassis, still not an issue though for your problem.

It's entirely possible the switch itself is cooked---not uncommon on our E-Series for some reason. Sadly that means digging into the dash trim to gain access to the pod were blower switches are housed. Once that pod is out it should be fairly obvious if the switch or its connecting pigtail lead have melted or otherwise at issue.

Also FWIW factory service manuals will be of little help unless you also have an EVTM on the way. That's the factory book of schematics, component location, connector pin-out locations and a few component testing procedures shown----an invaluable tool for us DIY's. I've found a number of them through eBay.

In the meantime you can visit AutoZone.com to view or download free schematics----you do have to "register" first---painless and quick.
HTH

Hmmm that's definitely food for thought.

My manuals also come with the wiring book so I should be good once they arrive.

In the meantime though, I'm pretty sure this was always a cargo van. It was originally purchased by a mining outfit and used for carrying samples... it was spray foam insulated and had a plywood floor when I got it. No rear seat mounts or anything to suggest it was ever a club wagon or passenger van. It had no rear headliner... just 1/4" plywood walls and ceiling.

Now I'm curious why they would have added that switch to the van unless maybe the factory one died and this one was the only option in stock or something... I guess it doesn't really matter because I don't care too much about getting rear control over the heater, but I figured it might be contributing to the no-heat issue if there was a controller tucked away somewhere but I haven't come across one and I've totally gutted and rebuilt the interior of the van already.

I'll start digging into the front switch to see if it's the issue... if not, then I guess I'll wait till the Ford books arrive so I can really dig into it.

I didn't ask if the rear heater was the factory type or aftermarket?

That control pod you have was almost certainly added after manufacture as there were/are two versions; with and without rear control. When in Rear Control position the rear blower speed is set by those controls, it not being adjustable from the front while in that mode.

Its good you'll have your schematics----those are invaluable for all the factory wiring.

Hope you get this sorted out easily and cheaply too.