Up towards the top of your screen there should be a navigation bar. Click on "User CP". Under "Settings and options" there is an option for "Edit Signature". That will allow you to edit your "signature line" for all forum posts. You can see mine down below with cars listed.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]