Tedious electrical problems

I purchased an 03 gt convertible last year with 10,000 original miles, garage kept, driven seasonally. I put it away for the winter— occasionally running it and also used a trickle charger. Ever since I started daily driving it this past spring I’ve had some electrical problems that I hope someone on here can help me with. Whenever I have the car running with the headlights on and roll down the window the lights will dim; if I turn on my rear defroster, the radio starts to cut out. I did a load test on the battery and it all checks out. Any ideas would be helpful, thanks.