1) I assume you are talking about an "XTL5000 Consolette."
2) Wiring to a headset should not be a problem, but neither is it as simple as might appear.
3) Step one is to figure out which boards are in your XTL Consolette. Most of the time you'll find either the L791 AIB (Audio Interface Board) or the L146 TRC (tone remote control board).
4) Step two is to program the boards, which you do via DIP switches after exposing them. For the AIB, you'll want to set S101-6 to OFF (local desk mike present). Assuming your headset loads to 600 ohm, be sure that S101-4 is set ON and S101-5 is set OFF (both are the default settings). If you have the TRC board, same settings for S101-4, -5 and -6, and S100-2 should be set for ON (local mike not routed down wireline) (note: if you are actually using the wireline, this may change, but at this point you should probably consult with the techs who maintain your wireline system). I'd recommend you set S100-5, -6, -7 and -8 for OFF, ON, OFF, OFF, respectively; this will disable EIA mode select so you don't see any unintended mode changes as a result of stray signals.
5) Lines for connecting your headset are on Accessory Connector #2:
Pin 14 is deskset RX+
Pint 15 is deskset RX- Note: do not short either of these pins to ground, or connect them to any device that will route them to ground, or you will damage the radio deck's audio amplifier! Also note: you may see pins 1 and 16 labelled as Rx lines; these are for TRC devices, not deskset devices.
Pin 3 is deskset TX+
Pin 11 is deskset TX-
Pin 10 is PTT, active low.
Pin 2 is ground (analog or digital) and can be used with PTT function if needed or to bond with headset ground.
If your headset performs audio muting and has a VOX circuit, use it. AC#2, Pin 25, which Motorola calls "Spkr_Unmute" can be used as a COR line in theory, but I've never found it to be reliable with third party devices.
6) Please don't take this the hard way, but I'm not sure this is a DIY project (unless you are a tech). If you do attempt this on your own, it might save you some headaches if you get your hands on Motorola publication 6881098C21-O, which is the install manual for the XTL Consolette series. The manual will also guide you as to any programming changes required to implement the external device.
Good luck.