MR units are ALS vehicles (4 door pickups) for areas where there is high medical call volume. MR154 is a PHX unit with PHX markings and personnel being run out of Glendale station 154. My understanding is that it was put in service to ease the amount of times E154 had to respond to the city for medical calls. Someone else may have better info but that’s my take on it.
Due to how busy Glendale FD E154 has been with medical calls into Phoenix, and with a lack of funds to build a new Phoenix FD station, a unique pilot program where a 4-person ALS medical response unit from Phoenix is being housed at the Glendale station, has started...
If you Google "Phoenix Fire MR154", you'll also find a Fox10 article about it, as well as other mentions.
There are at least a couple of MR units around the Valley. Phoenix FD MR918 runs out of FS3 during the day, and FS18 at night. Surprise FD just put MR305 into service, I do believe.