At the link below, I recently posted a schematic for a Kenwood speaker-mic that goes into a Kenwood HT.
A direct male-to-male in this same configuration works with the 2.5mm and 3.5mm jacks on the front of the factory intercom. (Note -- the speaker mic schematic has many features you don't seek and that can be ignored in the context of your question -- you just need audio.)
On an LT, PTT is available via the cable/connector under the left side handlebar cover. (Referring to my model year, 2006; I assume many others, but can't speak for all years or any other model as I've never researched it beyond my needs.) A BMW acce$$ory switch is employed with this. Be prepared to bring money (or make home-brew alternate arrangements you're happy with there), but, I'll observe this ... the factory switch sure is clean and easy to install and use day-to-day. Besides, if you own an LT, what's another 82USD, right? (Note - the switch may be configured normally open or normally closed. NO is typical/customary in this type of service. Looks quite easy to change should you get it backwards for any reason.)
Some have also wired into the CB jack underneath the Tupperware for all functions you've asked about. Never tried it, but, I've seen a schematic in the "file" section to pursue that approach as given here:
Conclusion ... the mic and speaker interface to the front intercom jacks as shown in the speaker-mic schematic. PTT may be accomplished more than one way; I use the cable underneath the left side handlebar cover in concert with the available "factory" switch.
Please verify all of this before permanent commitment to the given pin-out or using any of this information. Hope this offers you a launching point to access audio and switching so you can get your Yaesu up and running.