I added this chip to the CHMSL on my '87. It fits inside the housing and, when you apply the brakes, it flashes a quick 4 pulses, then stays solid. A quick and easy install, and adds a great bit of "peace of mind" when braking (along with the addition of a redundant mechanical brake light switch, which is the subject of a couple of other recent threads).
https://www.amazon.com/gp/product/B00SQEI4GO/ref=oh_aui_detailpage_o09_s01?ie=UTF8&psc=1