Does anybody know how Roy Thomas Baker produced the thick percussive rhythm guitar sound in the intro of Just What I Needed? 

Same technique they used for those fat vocal harmonies: double- or triple-tracking.

Seriously?  That's all it was?  That's too simple.  I thought for sure there was going to be a series of processors to get a sound that chunky.

I think the most important factor is they had great sounding equipment. I think half the battle is won by optimizing the source material so that what you are recording is superior sounding to begin with. it sounds to me like the Cars intro starts with one guitar with a bit of slap delay or quick reverb panned mostly center then the accented notes are definitely at least double tracked and panned more to the right with a snare doubling the accents panned more to the left and it sounds to me like maybe some guitar reverb on the left as well. I'm just guessing but that is what it sounds like to me. I love the sound of that whole album!

From an interview in Guitar Magazine.
So many of those songs—“Just What I Needed,” “My Best Friend’s Girl,” “Good Times Roll”—are now guitar-rock classics. What was your setup in the studio?
I only had a few guitars at that point. Because we hadn’t really made any serious money or anything. My main guitar was a ’77 Les Paul Standard that I bought at Manny’s on 48th Street [in New York City]. And I had a Fender Telecaster that had a Bartolini [Hi-A mini-humbucker] pickup in the neck position. I also had a Martin D-35, which you can barely hear on the record, but it’s there a little bit on “All Mixed Up.” And I had two effects. [laughs] That original Roland Chorus Ensemble, the grey metal one, and a Morley Echo-Volume for delay. And that was it. That was my whole arsenal!
 
He doesn't get into recording techniques in the interview, but did state that he was ill when they arrived in London to do the album, and did all of his tracks in a day and half. The whole album was recorded pretty quickly, as they had been playing all the songs live for over a year.

See the video I posted in the CH about the 1950s studio. You don't actually need all the cool gadgets we take for granted today to make a great record! Just patience and imagination.

The Cars' Heartbeat City was the first cassette I bought with my own money.
I'm off now to give it another listen (but not on cassette).  Thanks for reminding me of these classics!

Slugbaby
The Cars' Heartbeat City was the first cassette I bought with my own money.
I'm off now to give it another listen (but not on cassette).  Thanks for reminding me of these classics!

I had that one, and a few others on vinyl.  I used to record my own cassettes to listen to in the car(s).  No pun intended!