Hi!

Every time I put a cassette into the tape player in my
truck, I get a message on the led saying "TP CLEAN".

'What does it mean?

It means that you need to clean the tape head in the player.

How this is sensed depends upon the quality of the tape deck. Many of
them have a very simple timer circuit that turns on the "cleaning"
message after a specified number of hours have passed. It should be
possible to reset this by pressing a button, disconnecting power, or
running an appropriate cleaning tape on the deck. If you have the
manual for the deck, it should tell you what to do to reset the
warning.

Some higher end decks might have a way to sense that the tape head is
dirty, but this would be somewhat unlikely in a simple vehicle-based
player like yours.

If you do not have a cleaning tape, you might be able to use a cotton
swab dampened with isopropyl alcohol (90% or better would be good) to
clean the tape head and transport components (rubber pinch roller,
capstan, guides, etc).

It isn't likely that failing to heed the warning will cause any
lasting damage to the player or your tapes. However, tapes you play
may not sound as good. Build up of "tape crud" on the parts that come
into contact with the tape could cause a tangle or snarl down the
road.

William