Why are you assuming it is a spark plug now? And an exploding one at that? It seems you were spoon fed a buttload of misinformation and are now trying to work around that.
Nuke all that crap from your mind right now. Go pick up a Haynes or Chilton's manual for the Supra and start reading. Serious.
It's probably something really dumb like a forgotten ground wire, or hell, maybe they didn't put the timing belt on right and it's timed incorrect. At any rate you need to rediagnose the car yourself.
When you turn the key on and then crank it, what does it do? Are there any knocking or tapping noises coming from the engine? Does it spin over fast or normal?
Need YOUR diagnosis, not that silly lady and he half-wit mechanic's.
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