If there's lots of it on the surface, rub it with ice or pack some ice in a ziplock bag and tape it on for ten minutes or so to get it around 0 deg. Then quickly use a butter knife, credit card, etc... to scrape/flake off what you can in little chips and chunks.
If it was hot hot hot when it got on there (not likely, I'm guessing, in winter) then the sap will have been more viscous and will have absorbed more into the fabric. That's gonna require some professional help. You could hit it with some 'GooGone' for a sweeter smelling solution for getting it "out" of the fabric, but I'd personally not put much faith in that stuff.
If it was me, I'd use denatured alcohol (or methyl hydrate, same thing). Get a litre/quart of it at the hardware store for a buck or two and apply liberally. It WILL break up the remaining sap and you can blot it up with repeated applications of a clean paper towel. Don't rub the crap out of it, just apply the methyl hydrate with a sponge or paper towel saturated with it, let it sit for a one-two count, and dab it up with the clean paper towel.
This will get it out, or I'll send you five dollars. Caution though, I haven't put it on my seat fabric, and it may discolour yours, so I'll add the caveat on nearly every bottle of cleaner out there... "test in an inconspicuous area first".
If you use methyl hydrate, it will only smell 'alcohol-ish' for a little bit. The stuff is mega-volatile and will evaporate out of the fabric quickly without leaving a trace. That said, don't be smoking while doing it.
