if the harness looks bad and the wiring was frayed ,yes replace it -that may be the issue with the blinkers being slow .
did you check all the fuses both inside the vehicle and the ones under the hood .
here is a wiring diagram for your vehicle .
it doesn't show what all the fuses are for but it may help you trace down some things
usually in gm's its a grey wire for dash lights that supplies the voltage
I would start out checking all the fuses -as this is what I normally do -in case there are fuses that are bad that you didn't know about .
from what I can see in this diagram is that there is one wire that appears to go from the under the hood fuses to the interior fuse box .
usually the dash lights are on a lower amp fuse -most times a 5 or 7.5 amp fuse .
I would replace the trailer wiring too..
check for voltage on the grey wire at the interior light dimmer switch and the orange wire should be hot
the grey wire should show voltage and decrease when you use the dimmer to dim the dash lights