Yours or the one Charles posted on .org could be the same as what's on this '89 SMPV (shows up better in the full-size photo):
Here's another '89 SMPV and a different style light:
http://www.sspmustang.org/alex/smf_photo_AG_5.htm
This one has the shroud around the light and is more like the ones that were adopted when that light became a standard item. BTW, this SMPV is still showing up in the smog test database so it's out there and being driven. But I just discovered that an identical blue '89 to this one was destroyed in a hellacious night wreck that almost killed the officer. He was responding code 3 and passing stopped traffic on the right shoulder of I-5 north of San Diego when he rear-ended a parked car with no lights on.