1998 f150
A little bit of homework here. This year (and up to 2006, on certain Fords) have a single 16 pin rectangular connector. I have a '98 Lariat, and this was one of 2 years that had an external amp in the center kick plate, resulting in 2 connectors, and I had to use the RCA jacks out of the aftermarket unit to make it work. I suggest you get the radio removal kit, which is just 2 U-shaped clips, from an auto parts store, and physically inspect what connector the truck has. Then look at what connector it has. THEN, check Ebay for your choices, as they'll often take a picture of the back of the unit as well. This will allow you to choose what works for you. There ARE newer models (CD and Cassette) that will fit (just avoid ones that have more than the 16-pin connector), however, they'll ALL be too old to have a 3.5mm mini AUX jack, or any other feature. As me1234 posted, for $5 or so on Ebay, you can get the cassette adapter that has that 3.5mm mini jack.

