18 gauge will be sufficiant . as far as i know all eclipses have 6 speakers even without the infinity . your best bet is to get a set of components (mids,tweets,crossovers) and hook it up to a 2 channel amp . rear speakers are bad for the sound stage of the car . most pro installations do not use rear fill because it moves the staging to the rear of the car .

LED tails : Done for now!
99 eclipse rst
cobalt blue met. paint,star stage 2 upgraded t3/t460trim turbo ceramic coated , polished compressor housing, ,optima yellow top, chrome i/c piping ,front mount ntercooler ,apexi turbo timer . zeitronix wb02 ,faze oil ,boost gauge,egt ,bosch 4x plugs,kyb agx struts, pro kit springs.17''adr venomous rims,Falken azenis ,brembo dilled/slotted rotors ,turbo xs bov ,oil catch can,manual shiftknob,autometer pyrometer egt , vortech sfmu,b&m fuel pressure gauge ,accel 310cc injectors ,gasketmatched and polished intake manifold ,58mm throttle body ,no egr,polished turbonetics wastegate,fuel pump rewire,homade fcd and a lot more .LED reverse lights ,sidemarkers and tails .atx to mtx swap . South bend tz clutch . Tons more
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