Cops are incentivized to keep their mouths shut, and ignore the corruption within their ranks. If they bust each other, they risk losing their pensions if they embarrass the department bad enough. This is just one reason why we need to put them into something like 401k plans rather than pensions.
There need to be other incentives to get rid of the corruption, though. Cops who bust other cops are ostracized, harassed, threatened, etc. They need a good reason (and some anonymity) to report misconduct within their ranks.
How many bad cops could we weed out if each state had a yearly million dollar prize to a random cop who outed a bad apple? If they could do it secretly, even better. Call the tipline, and the tipline calls the local chief to let him know that the problem will be reported to the media if it isn't fixed. Every time that happens, the reporting cop gets an entry into the prize lottery.
There's some good reading here:
http://www.policemisconduct.net/