Oct 29 (Reuters) - Amazon.com Inc (AMZN.O), an early backer of Rivian Automotive, now owns about a fifth of the electric-truck maker, the e-commerce giant disclosed in a quarterly filing on Friday. Ford Motor Co (F.N), which has a strategic relationship with Rivian, removed a representative from the startup"s board earlier this month. read more At the end of the third quarter, Amazon had an equity interest of about 20% in Rivian, according to the filing. Rivian is also aiming to raise between $5 billion and $8 billion in a stock market debut later this year, and seeking a valuation of about $80 billion, Reuters reported in September. read moreThe company"s pre-IPO investors also include T. Rowe Price (TROW.O) and BlackRock (BLK.N). Amazon on Thursday reported lackluster quarterly results and warned that a tightening labor market and rising costs would hurt its holiday-quarter results. read more