Composting toilet electric fan

I have spliced the electric line into the water pump line. When the water pump comes on the fan stops working. Most of the time it will not come back on. Any ideas of what is going on?

Loose connections, overloaded circuit. ( wire to small) for the load. Checks grounds. Are lights for a specific voltage only (12vdc) and not a range (10-28). When pump runs it could run down the suppply voltage to the other loads on the circuit and out of operating range for the lights.

How exactly did you “splice” the wires? Possibly incorrectly if you wired it in series. Or dragging down the power supply like Trav stated. Take pix if possible.