Generally there is no problem.. Yo umay wish to visit www.crutchfield.com and see if they have an adapt-a-kit.
Also if your old radio has a CASSETTE input, that works as an AUX in too, though not as good as the real deal.
Some radios DO have aux in capibility but you need a plugs into the back adapter. (My Dash Radio for example).
and finally there are many FM_transmitter options, Radio Shack carries a few of them and if you google Radio Shack Stores Closing.
The prices at the stores on the list are kind of good.. Very good in fact.
NOTE: FM transmitters can be a problem in heavily saturated markets,, No blank space on the dial or your transmitter.
There used to be one company (I think AudioVox) that made one that went inline between the FM antenna and the radio,, That fixed that, but alas Have not seen it in more years than I care to admit to.