Cowl vent floor pan trunk rear wheel wells pretty much anywhere its a old car and rust is like a crazy ex always showing up in the wrong places.To test the cowl vent its the holes by the windshield and wipers pour water in it if it comes out in the floor pan of the car its rusted out.the cowl vent is a real mother to fix gotta take the windshield out and cut 120 something spot welds.Also take a magnet off your fridge try sticking it around the rear wheel archs if it don't stick then theres thick body filler hiding rust or a wrong repair.Also take a hammer and give the frame rails front and back a good wack they can rust from the inside out so if the metal is thin you'll find out.
I started from the bottom up when I went looking at mustangs. Put your finger in every hole of the floor pan you can, there are rubber plugs over the seat bolt holes and others, if you can pop them off feel around for piles of rust scales, in the front/rear frame rails also. If the seller lets you pull up a corner of carpet look there too. Torque boxes and cowl grill area as mentioned above.
I found a car with some rust in the outer wheel well areas, that can be fixed when the body work/paint is done. The car looked new underneath with very little rust, I don't weld so that was important to me. Stick you hand down to the lowest point of the trunk fender corners you can and lift the trunk mat and look around the gas tank mounting area also.
Basically crawl all over the car but water settles at the bottom of everything. I looked at a conv. and the first bad sign was a new paint job, pulled piles of rust out of the floor pans/frame rails, when I shut the door I could see the rear quarter move...
Didn't buy that one.
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