If the connection is not making good contact, your voltages will vary with electrical load, or current, demand.
Bad electrical connections put all sorts of interesting voltages on the car's electical system. "Interesting" voltages confuse computers, which then do strange things!
With heavier current load, like when you turn on your lights, the voltage drop could be enough to dim them severly.
Is the battery terminnal clean? Did you get the connector back on correctly?
Try removing the connection, cleaning the post and connector and re-connecting. Don't over tighthen, but make it snug. Gently wiggle the connection when you're done to be sure.
I hope this is all there is to it. Let us know.
'96 E300D 60k mi (wife's daily ride)
'95 Audi 90 120k mi
'92 GMC Suburban 139k mi
'85 300SD 234k mi (my daily ride)