Not sure if this is an issue with the SL, but on the 210 chassis and the SLK's, there was/is an issue with the brake light sockets being bad and/or losing contact. A search should bring up info on that. Also, are you sure that the license plate lights are okay and not losing contact? Are there any lights actually out when the indicator comes on? Check closely - those license plate lights are often overlooked. I sure wouldn't jump at a new control module just yet.

__________________
Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles