Renault kangoo 2001 electrical short on braking may be earthing?

I have a 2001 1.9 diesel kangoo.

When I brake, the dashboard lights (like I had the headlamps on) and the left dashboard indicator display light also goes on. If I happen to be indicating left at the same time, the indicator ticks much more quickly (as if the bulb had gone). The light cluster looks OK but there is a crack in the outside shell over the brakelight.

In addition to all this, the battery goes flat when it rains heavily.

can you help me track down what appears to be a short circuit?