When we recently acquired 2003 S-Type, on the drive home my wife (following me in the "old" Malibu Maxx) noticed that the LHD rear turn signal was a plain, white light, and very bright. Sure enough, the PO had used an 1157 equivalent in there, which shouldn't have worked as it is a dual filament bulb.

When I checked the Electrical Guide it specified a PY21W as the replacement. The Owners Manual says 3457W, which is a plastic wedge type. I found that the correct bulb is a Sylvania 7507, which can be found in any parts store. The Sylvania site has a nice catalog function to enable you to check any year or model: OSRAM SYLVANIA - Replacement Guide - Start

The 2003 Sylvania listing is attached.