Best thing to do is pull the dash apart and inspect if from the inside...like the link Kunar posted, they rust out around the windshield support...
You may, however, have a similar problem to mine...my windshield is not sealed properly...but the only way to find that out is to pull the dash...and that's not actually as difficult as it sounds...
There are about five screws and a few bolts that hold the dash in...(once you take off the trim and other stuff) and they are pretty easy to access on an 88...but keep a small bucket handy and take pics of what you take off and where it comes from if you get confused easily because you will end up with a small handful of screws and bolts once you've taken it apart...
If there is no rust on your firewall or around the windshield supports then you can try the water spray since you will be able to trace it much easier with nothing obstructing your view...
Hopefully it's just the windshield seal...because the firewall is a PITA to repair, but it can be done if you really want to keep the truck...I drove mine for about two years after I discovered some major rust...only stopped driving it when the engine crapped out on me...