Since we are talking full-range speakers, it is always best to fit the biggest speaker you can in an available space. The early Wranglers got stuck with crappy little 4x6-inch speakers in the dash and the JKs got a 5¼-inch speaker, which is better but not great. A 5¼-inch speaker can be made to fit the TJs with the shown bracket and a similar conversion is available for the JK to go up to a 6¼-inch speaker. The foam cup is a speaker baffle and does a lot in a Jeep to keep the speaker working. It keeps dirt and water from getting back there and provides the acoustic suspension that helps keep the speaker from bottoming out when driven hard. The baffles are Crutchfield PN 237XT55 and the adapter is 4WD Hardware PN MTCJTJ514B.