It's a 168 wedge or 194 Wedge. A W5W bulb will also work. In order to replace it you have to pull off the grill, pull of the grill side pieces and then remove the headlamps.
Have you registered your Forster on http://my.subaru.com ?? It's free and it also gives you online access to your owners manual in .PDF format. Once in there, just look up bulbs and it'll take you right to the install page.
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