You need to do try some things....
Ground the battery really good to the unibody then find the two ground wires going to pins #40 and 60 plus the ECU relay ground wire ...
Those wires need to be cleaned up and grounded good to the unibody too but best place is to route those negatives direct to the battery as the battery is a Cathode that absorbs r/f noise.....
For the negative path in a trunk mounted battery I do whats very unheard of as its not too cost effective but there are cheaper ways that work just as good using this stuff.... ....
For my ground strap I use a roll of 1/8" thick soft copper flashing and use a brake and a sheetmetal trimmer and I trimmed off a 8ft long piece thats 2" wide then I made two overlapping bends by folding it back against itself then soldered the joint...
After that I drilled 40 holes in it and then I took a grinder and I grinded away the paint on the inside floor of the vehicle 1" then I drilled and used copper rivets to rivet the copper strip to the body all the way to the firewall then I made a copper grounding lug that gets attached with a thick braided grounding strap to the engine block and another 2 gauge cable to the core support area...
I also take some paint then some undercoating to cover over the area then I put back the rug and the passenger seat if its a full interior vehicle...
I even made special grounding strips for my regular batteries that filter EMI radiation aswell as r/f interference..
If anyone knows physics and such you'll know that it takes destructive methods to destroy EMI radiation until it was discovered in 2017 that Ansitropic Crystals absorbs EMI radiation naturally....LOL.....Oh and for those that dont know the alternator produces EMI radiation when it produces voltage....
Some will say that solid copper grounding is not needed and plain old cable direct to the unibody is ok but you need a double run of 0 gauge to ground an automobiles electric system better than solid copper or the steel unibody when the battery is further than it is from the factory as length adds resistance so youll need a battery that delivers atleast 1000 cold cranking amps and 850 cranking amps and a ground that bridges the power lightning quick and stong as an ox..........
I would also run a 0 gauge positive wire and if you dont have a mini starter mount the starter relay as close to the starter mounted to the transmission or frame....
I also run a 4 gauge wire from the battery to the two column switch power feed wires and add a 4 gauge wire to the 10 gauge ECU relay power wire too...
That much extra distance will cause all sorts of voltage issues...
In my Lamborghini Diablo kitcar I made the whole firewall out of a 1/8" 4' x 8' sheet of copper too with a copper busbar from the battery to the firewall...........It wasnt easy but it was well worth it as I dont get any of the power issues the other members get with slow lazy power windows or slow cranking highly laborous hot start issues or dim lighting on the instrumentation panels and most overall;-No Stacked-up grounds all on a single screw because fiberglass is a very poor electrical conductor ...LOL