Battery draining over night

I recently replaced the battery in my car due to age and the fact that the brake lights kept coming on during the night during the winter. On closer inspection I found that the switch needed adjustment and the problem was solved. I thought. Now the lights no longer come on at night but the battery is drained and needs jump starting most mornings. Problem now is that the drain is so bad that I need to disconnect the negative terminal just to be able to start in the afternoon after work. Struggling to find the cause of the drain. Please help.