Weschler is buying about 20-25% of the volume, which means he is buying as much as he can. His purchase spree has actually already driven up the stock as it started to change direction right when Weschler entered the market.
He started right one month after the merger, given that he was probably restricted as an insider/10% owner. BRK now owns 32% of SIRI.

The SIRI story is actually quite simple: Over the past year or so the company remodelled its range of subscriptions and pricing, invested in a new app, analytics, new satellites and in the meantime reduced marketing spending. At the same time, car sales were down, which reduced the potential subscriber funnel. As a result, they lost some subscribers (surprisingly little, I believe, given the situation).

SIRI trades for less than 8x FCF, which is heavily depressed by the above mentioned temporary effects, in addition to one-off merger costs. It will certainly grow again to the $1.4-1.5B level within 2 years. It currently trades for $8.5B and pays an over three times covered 4% dividend.

The market is concerned about competition, an audience skewing older, debt (temporarily higher because of the Liberty transaction, which effectively was a buyback), and there was lots of technical noise around the transaction.