Start with a hose at the bottom of the back glass and slowly work your way up. It helps to have the carpet and perhaps rear seat removed. Rust under the rear glass gasket can go into thr trunk and sometimes the passenger compartment.

The door seals can leak. The only way to find is to decide to be thorough. I always try to short cut then take it all apart when the short cut doesn't work.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.