my 05 S.MANN cabrio did exactly the same thing...MIDDLE OF WINTER OUT IN NOWHERE-LAND..
here is the fix... remove the battery ground cable off the negative post FIRST... buy a 2 ft long ground cable attach this from the body frame stud (rear left side ) remove the old clean the base with sandpaper or wire brush, place the new on the stud and the OLD on top, tighten the nut, the other end attach to the bolt next to the DIPSTICK . clean and tighten that one...replace the battery ground cable..
try it... if it goes away its fixed..(thank me)
if not do this next.. jack and secure the rear drivers side off the ground, remove the rear wheel, pull the 9 pin connector off the gear box, spray contact cleaner in it and let dry.
replace and clean the harness and surrounding area .. make sure agian the battery is disconnected.. smear dielectric grease on the inside of the connector, put it back on..
attach the battery back...
TRY IT... if it works .. (thank me)
if this does it then smear silicone sealant around the connector while connected sealing it completely... your good to go!..
if none of the above work, disconnect the battery for 24 hours, replace CLEAN...