A quick Duck-Duck-Go search for 1988 G-van cluster turns up this image. Looks like it's in the small lower center circle, below the gas gauge. Yellow words at about the 9 o'clock position that say Service Engine Soon.
First things first. Check the bulb. If it's good, wiring. If that all tests out, you may have a bad computer. With terminals a and b shorted together, the computer should go into a self-diagnostic mode, where that light flashes a code 12 to tell you that the self-test and bulb are working. Then it will flash any stored trouble codes.How To Get OBDD1 Codes