If it's in the hundreds of milliamps, that may be the problem if the battery runs down in just a couple weeks. A poorly designed or installed car alarm system may have the same effect. You can check the standby power drain on the car by inserting an multimeter with a sufficient DC ...
Once you find the short circuit, then you can go about repairing it. Before reconnecting the battery orputting in a new fuse, recheck for short circuits with the test light or multimeter.