The pedal/throttle position switch is located at the center of the brake booster cannister, just under the fuse tray area in the engine compartment. The cannister is large and black, the sensor looks like this:

http://www.fcpgroton.com/category-exec/category_id/154/nm/Fuel_Injection/sub_top_menu_item/by_make-by_model-by_year/by_make/78/by_model/1137/by_year/44

As you may note, the knockoff is much cheaper than OEM but in your case get the knockoff. First inspect the wires that go to the sensor, make sure the plug isn't loose or dirty. It is always a good idea to clean the plug before buying a new sensor. The ABS light will turn off after you get to 18mph if the sensor is working.

Because you don't get full boost, I suspect that there is still a leak in the intercooler piping, somewhere. Getting 26mpg, combined, is great news!! That means that the engine is happy and your wallet is wearing out less.
One note, if you are using regular gasoline, 87 octane, switch to 89 octane as a minimum. The engine will not go to full boost on 87 octane because of detonation problems.

Klaus
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Volvoless. Sold the R. There is now a void...