Well, i think the fresh air comes in from the cowl, under the windsheild wipers. When you press recirc, it aparently comes in from the passenger side foot well. I would'nt know how to make a filter for that thgouh.
one observation of interest though, when my truck was new, i would press recirc and would'nt smell any outside air, exhaust, farm etc. But as time went on, i began to be able to smell exhaust and stuff even with the recirc button on. I would like to know how to revert the flap or whatever it is that closes the fresh air intake, and make it like new again.
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