On my 2004 supercab it is under the back seat on the passenger side. Hope that helps.
Same goes for my 2007 Supercab.
I'm sure you could also consult the manual.
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2007 F150 FX2 Sport SuperCab 5.4l, 3.73 LS, 6.5' Bed
Magnaflow 16616 Catback Side Exit Before Tire
S&B Cold Air Intake
VMP Tuned XCal3
McGaughy's 2/4.5" Drop Kit
McGaughy's Rear Airbag Assist Kit w/ 3 Gallon Tank, Custom Made Control Unit
22" Boss 311 Rims with Toyo Proxes ST's 305/40R22
Leer 700 Series Tonneau, BedRug Carpeted Liner
Hella LED Tails, Hella LED 3rd Brakelight, Line of Fire LED Brakelight Strip
2 door is either in the rear door panel (the small door that opens) or just sitting there obvious. can't remember how my friend had it in his truck. supercab and supercrew like everyone said is passenger rear seat really obvious. if you dont see it you dont have one.
Same goes for my 2006 scab, right under the rear passenger seat. Definetly obvious, so if you have an scab or screw and don't see it, like 06yz250f said, you probably dont have one.
Does anyone happen to have a picture of the jack located under the seat? When I bought my truck, I noticed it was removed. I have found one on ebay but I want to make sure it has everything I need including the hold down tray.