What I think needs to be opened isn't as much the source, but a transparency log to enable the public to audit advertisements as they would be publicly available. Perhaps crowdsourced tools to scan new ads on each platform for problems could improve ad quality.

Similar to how modern X.509 Certificate Authorities are implementing Certificate Transparency (CT), I think the advertising industry needs to do the same kind of thing on its platforms, especially the ones processing thousands of ads. Under such a transparency system, each ad platform would maintain a standardized log where every advertisement entered into their system is logged and made publicly available without restriction. If a website runs its own ads, it should run a transparency log as well as it is operating as an advertising platform.

Each log entry would make available a URL to a "staging location" where the ad can be reviewed on demand by anyone or any tools and the office contact information of the submitter (by name, company, title, address, phone number and email address), among other metadata and of course, entries would be cryptographically signed to resist tampering.

It'd be easy for browser add-ons to search the transparency logs and enable users of websites to trace accountability for each and every ad back to the very person who submitted the ad, or in the absense of the log entry it could identify ad platforms that have "gone rogue" or were compromised and are a high risk. It could also encourage ad networks to be more strict and to verify submitters more closely, and also enable people to correlate the quality of ads (or lack thereof) with specific agencies.

If that isn't possible "due to legal issues", then the industry as a whole is protecting corruption, plain and simple. It's time for the ad industry to earn the public's trust, failing that, the public is perfectly content with continuing to block a secretive industry.