My heater/ ac isn't blowing as good as it should. Where is the evaporator on the astro?

From under the hood, it is inside the large black housing that is attached to the bulkhead on the passenger side. Are you able to get all 4 of the blower speeds?

I am only able to get the lower 3.

There is a relay that controls hi speed only. I believe 2000 is in the underhood fuse panel. There is a schematic on the underside of the panel cover that shows the relay location. The relay or it's wiring, or even the switch may be faulty. Hope this helps.

Your heater fan speed relay is located in the plastic box just below the fan motor. You will need to pull out the windshield washer bottle and the antifreeze over flow bottle, the box is just behind the bottles. pull off the 2 screws and the relay is simply a pull the old one out and push the new one in. They are a common part to replace. If you are having problems with the amount of air being blown, this might be due to the air not going to the right place. There is a vaccuum hose that goes from the left rear of the top of the engine to the front. the hose is about 1/8 of an inch, It burns out on the manifold and breaks limiting the air flow selection. I fixed mine a couple of times on all of my Astros and Safaris.