Eh, okay, how about "speakers providing the correct impedance"?
Yes, I was not technically correct, but I think that matching the numbers on the little stickers on the backs of the devices would suffice...

When I used to work at the junk yard (best job ever), during crushing season we all ran Front End Loaders moving cars to/from the crushers and trucks.
Being on that thing with no music was brutal, so I rigged up my own system, however, probably not the most tool box efficient.
I duct taped a car radio to a car battery and stuck a speaker (using its magnet) to the top of it. Wired it up and had a portable (though very heavy) boom box for nothing.
Just thought I'd throw the advice out there for any fellow rednecks with no pride in the appearance of their stereo.

another ex-junkyard worker here. i worked at pick n' pull processing cars on the drain racks. processed approx 40-60 cars 8 hours a day, 5 days a week, all year long. lot of cars came through, and they all had interesting **** left in them. i built a stereo for the shop building i worked in entirely out of components found in cars. had a rack unit and speakers strung all over the drain rack. it was loud! you could hear it out in the field. that was a fun job. got paid to break sh!t.