My heater smells really bad, and I have taken it apart far enough to reach inside to the bottom of the plastic heater box and feel antifreeze. So it needs to come out.The issue I have is how to get to it. There is small panel that comes off easily so I can see part of the core. I can also see a vacuum actuate flap that closes off air flow to the core.At this point, is the best plan of action to remove major pieces of plastic and/or dash to get at this? Or is that needed? I haven't found a step by step removal how-to for the 1st Gen D250. But how-tos for other cars talk about removing a small rectangle of heater core box for access, then riveting a cover over the hole. Has anyone done this?Thanks in advance for your help.

Could you shot some pictures of the box and what you got so far and I'll try to guide you the rest of the way...

I took some photos. Please let me know what you think.

Thanks!

Ok... Start off by getting the blower motor and heat shield removed from the engine side.Then remove all fastners on the engine side.The box should be loose at this point and if the dash is free on the passenger side it should flex enough to allow the HVAC case to slide under the dash.

Thanks for the help. I got a little farther last night, and will get started again once I get some presents wrapped

Fortunately I have another car to drive to work tomorrow.

The heater box is still not free, so I may have missed a fastener on the engine side.

Once I can pull the box away from the firewall, can I get the core through the opening under the center of the dash, or does the top of the box need to come up and off?

Again, thank you Michael.

Now that I seen the a/C line you'll have to dump the freon out of the system. (Sorry to say). Then once those lines are pulled you can pull the HVAC box out from under the dash and kind of roll it out from under the dash with a sliding motion.Be kind of like this...

Dumping the freon would not be a big deal, as the A/C hasn't worked in 10 years. It is still and R12 system. I will prod the valve with a screwdriver and see if anything comes out.This truck originally came (in 1989) without A/C, and A/C was added by a dealer in 1992. Does that make a difference? You can see in the pics the sloppy cuts in the dash for A/C vents. It was a Canadian truck, with metric speedometer and no cruise control.This is the 2nd heater core that has gone bad. A shop replaced the first one for me, for $90 labor (I supplied the part). They didn't mention evacuating the freon.The part I bought has a lifetime warranty, so I wanted to do it myself. That, and the face that this core only lasted 5 years, so I felt like I need to know how to do it (if only Cummins made the rest of the truck!)I think I've already asked, but any chance I could cut an access hole and patch with rivets? I would like to get this back on the road. I apologize for trying to take short cuts, but this is a busy time of year. Thanks again for all the help.