Heater fuse keeps blowing.

Guys I’m going insane, I have a 97 Transport with the rear heater. If I start the van with both the front and rear heater on I have a 1 in 3 odds of blowing the 25 amp fuse. My meter is only rated at 10 amps So I have not tested the current draw but man have I changed parts LOL… Yes I started with the heater motor then the switches. Can anyone lend a tip or two for what I should be looking for?

I believe I have heard a click or two from the back when it blows. However it may just be a quittance. IS there a relay in the back ? Where??

I do not have circuit schematic specific to the 97 model..but based on my '93, front heater in HIGH does not draw current from the 25A fuse. Rear heater fan does though through a specific 25A fuse. Lower speeds front and high speed relay COIL front draws current from ANOTHER 25A fuse...which one is blowing?. High speed current to front blower is from high speed relay CONTACt and a separate fusible link.
You did not mention if fans were in High or other setting.
Depending on your answers, I'd suspect a shorted coil in low speed or high speed relay, or a pinched wire somewhere. Disconnect the front High Speed relay..not sure where it is on a '97...this will let everything else work and could provide some clue.

There is also a electrically operated valve on the coolant circuit to rear heater but it is only grounded through the heater circuit. It draws its current from another 15A fuse.

As a starter, you can look through these '93 diagrams: they might be very indicative of yours too.
www.avigex.ca/xport/hvac_diagram_front.jpg
www.avigex.ca/xport/hvac_rear_dual.jpg
www.avigex.ca/xport/hvac_rear_single.jpg

Thank you for the diagram’s they got me started. I came up with a good wiring diagram for this model and I have concluded that my problem may lie in the Auxiliary Temperature valve actuator’s logic module. Do you have any idea where this might be located?