Looking at your post it mentioned the ONLY light still functioning is the LEFT Front?
"I fixed it then went to recheck all my lights again to be sure everything else was still working only to discover that now the
brake lights,
the front and
rear right signal lights and the
left rear signal light no longer work
"
The 3-2 converter box appears to be a Buyer Products or Curt Mfg unit.
About $15-$25 most anywhere. Advance Autoparts, U-haul etc...
https://www.buyersproducts.com/produ...iteria=5423203
https://www.amazon.com/BUYERS-PRODUC...gateway&sr=8-1
https://www.delcity.net/curt?id=Towi...CABEgJ6M_D_BwE
Looking at the ERA wiring diagram page 15 the Pick-off for the front Turn and Parking lights occurs Before the Converter.
http://www.erareplicas.com/427man/wiring/wprimer.pdf
You didn't just convert to Led bulbs per chance? That can create the same scenario you are experiencing.