Signal sense widgets work fine in that car. PAC TR4. Solves that problem cheaply.
You don't need a line output converter. The 400/4 can take that speaker level input.
BUT... the head unit output is not linear, so you are amplifying a very nonlinear signal. If the signal coming out of your head unit were linear, this graph of the electrical output would be flat:
So you're gonna want to correct that or it doesn't matter what else you do. The Jl CleanSweep will correct it on the front channels AND do signal sensing...