Hey Strad, something like this will prolly be common with the Levin.
You've gotta remove the dorr trim to get to the speakers. Not very hard to do at all.
It's only a few screws to get it out, then a standard size (think it was 5 1/4", but may have been 6 1/2") will slot straight in.
Not much point in going up in size... you'll be cutting metal, and you don't actually need any bigger in the front doors. If you want to go splits, it's the same... but you've just gotta find somewhere to mount the crossover and stick the tweeter.
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