MidAtl has released their 2018 Supps. Note their section on rain lights. Need to have 2 taillights, one of which needs to be FIA approved. So that means either buy an FIA light and get a taillight working, or buy 2 FIA taillights. No “flashing mode”. FIA light must get brighter when brakes applied. http://www.nasaracing.net/wp-content/uploads/ma-supp-ccr.pdf
SE sponsor Harrison Motorsports will happily sell you an FIA rainlight. Afterburner FIA Rain/Brake Light with Harness & Weather Pack Connector
Related. FIA lights are pretty bright.
I’ve been talking to Jeff Hall for the past couple months about this issue because he was involved in crafting the MidAtl rule and I was trying to help craft the SE version of the rainlight rule. I swanned off to do some background research and found that there is no tailight/brakelight aftermarket bulb replacement that will get anywhere near the same brightness as the FIA certified lights. The certification has a specific min. brightness spec (1800 Lumens). I looked all over hell and couldn’t a replacement lightbulb that had even 1/3rd that much brightness.