Today I replaced the tail lights on my '54 Chevy 3100. The brake lights come on when I apply the brake. The turn signals and four-ways work like they should. The problems begin when I turn the headlamps on. With the headlamps on the brake lights do not work nor do the turn signals & four ways.

I thought about replacing the flasher, but have not yet done so. Any advice / trouble shooting would be appreciated.

Thanks