If you call Phil at Partshop/fastlane with your VIN number and year, he can get you the correct one. After you removed the cover behind the battery, you will see several small boxes. The OVP is closest to the firewall, and closest to the center of the engine compartment. It has a clear lid on a red top with a blade type fuse on the top. It pulls straight up, and is 'keyed' so that it only goes in one way. The fuse on the top may or may not be blown if the OVP is bad.
The rough idle may not be the OVP (fuel pumps, coil, plugs, wires/points, fuel filter, etc.) You can check the fuse on the OVP and if it is blown it is probably one of your problems, but if it isn't you may not have ruled it out. Check the least expensive things first, and good luck!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E