this may help ? not sure what year your working on ? :
most likely several places on one of the 2 boards need re soldering but also burned out resistors and capacitors . Moisture gets in the box > you can pry out the boards after bending the tap's back. They may be stuck in the Case some so take your time. Once out you can separate the boards from each other > Plastic pin clip
Repairing your Porsche 928 Central Warning System (CWS) controller
Link :
http://members.rennlist.com/tom928/Porsche_928_CWS_Repair_rev1p0.pdf
If your Warning is On all the time with the key on :
Try turning on the key after unbolting the box to see if the warning light go's out with the box pulled back from ground . If the light go's out most likely the cause is in the box . (1) The easy Fix is to try a know working
box (2) install a good box new or used . Someone on the forum may have one 1978 x 1983 box > ( number on the back of box perhaps like my 1980 > 928 641 602 .01