Type 1 Clarion Radio has 2 female connector outputs (5 pin DIN) that can go directly to intercom head sets or more frequently, one of the outputs gets connected to an amplifier to power the Clarion speakers. To use both, there was an optional switch available that mounted in the 3/4"rubber covered hole under the left speaker that would cut off the speakers when using the intercom to avoid feedback. Alternatively, I have used a modified Type II 6 pin to 2 x 5 pin Y cable, modified to 5 pin to 5 pin to 3 active pin and power cut off switch to achieve the same results but that is another story.
To tie in an MP3 you could build a Y cable with a switch to replace the radio input to the amp. The wiring would be pretty simple, but the mp3 would not make it to the headsets. I don't remember the color codes but the five pins can be tested to find: speaker ground, left speaker, right speaker, microphone ground, microphone input. The MP3 output should be 3 wire: speaker ground, left speaker, right speaker. The grounds can all be tied together. The four speaker wires can be toggled with a double pole double throw switch (DP DT) which is available at many stores for less than $10. The setup you mention for the type II units is probably similar to the one I just suggested, since that system also goes through an amp to the speakers except it does so through a 6 pin connector.
The easiest setup is to get one of those FM transmitter units and then you can just dial it in and keep all the functions of the Clarion radio, inlcuding headsets, running.