Fender is an old robot friend of Bender Bending Rodríguez's from high school. He used to work for Jack in the Box, but then got a job as part of the Beastie Boys as a stereo. He has a speaker cabinet for a head, with a large speaker as a mouth and two smaller "tweeter" speakers functioning as eyes. His lower half consists of a Fender brand combo guitar amplifier, which is where his limbs are located. This amplifier functions as his torso.
The animators didn't seem to understand how guitar amplifiers actually work, thus botching part of the gag, as Fender's setup is backwards from the industry standard for 2-piece rigs, which have the speaker cabinet on the bottom and the amplifier "head" (wink) on top. They also didn't appear to understand that his bottom half is actually a "combo amp" with an integral speaker behind the trademarked Fender mustard yellow tweed cloth (it doesn't move when he talks).