I have a 89 2500....was going to switch the gauges out from a 90 1500...but after i put the dash in i took it out and replaced the original but ever since then the break light has been flashing constantly...anybody know what would cause it.

A flashing brake light on that year truck indicates a problem with the anti lock brakes. The code will need to be read in order to know what is causing it and I am not sure if there is a way pull brake codes without a scanner.

have same problem. mine started after having short in ecm power wire.found this site to be very helpful as it explains flash codes.
hope it helps.
http://www.autosite.com/garage/encyclop/ency14l.asp

sounds to me like there is a pinched wire or loose wire on the back of the dash gauge cluster

I had the same problem with my truck after taking out th gauge cluster and replacing it. I didn't disconect the battery when doing and I think it puts a fault in the computer. I then disconected the battery for a few minutes and the connected it back, everything seems fine and no more blinking. Give that a try.

Try jumping terminals A&H on your ALDL, that will give you codes for your ABS if that is the problem.