I'm assuming you have no control line on your side to set the dataflow
direction, but that the radio enables its transmitter when required,
and sets it to HiZ when receiving. It's possible that some obscure
control character(s) or other RS232 line sets data direction, but let's
assume the arrangement is simple (does your manual say anything about
this?).

Simply tying the RXD line directly to the radio data line, and the TXD
line to that with a 3K or so resistor ought to work. Anything you send
will be echoed on your RXD line if you use this scheme, but under our
simplicity assumption it is likely the Kenwood cable does this as well.

Note that I'm assuming RXD is the input to the MAX 232 and TXD is the
output. RS232 nomenclature is ambiguous.

Regards,
Glenn Dixon AC7ZN