I think a P bass has a 0.047Uf cap,some basses have 0.10Uf, [a 0.10 is more "bassy"/filters out more treble] a reasonable quality Disc cap should do (just a little disc -no high voltage generated).
Where it should go depends on the wiring -some guitars have a cap from the high tab to the wiper and some have a cap in between the wipers .

Just to confuse you a little more, standard strat has a cap "as" ground - ie, low tab [usually soldered to back of pot] has one side of cap to tab -other to back of pot... like this..




Try it like that first.

Good Luck.