What is "Tachtav" referring to?
Rashi: It is referring (not to the what is underneath the Merkav, but) to the Merkav that is underneath the Zav, to teach us that, as opposed to Mishkav and Moshav, Merkav renders Tamei Adam, but not the clothes that he is wearing.
What is the latter half of the Pasuk coming to teach us?
Rashi #1: One who carries anything mentioned about a Zav - his Zivus, his spit, his Zera and his urine, his Mishkav and even his Merkav, he and the clothes become Tamei.
Rashi #2 (in Shabbos,82b): It is referring to Even Mesama - If a Zav is sitting on large a stone that is supported by four posts and touches Keilim underneath it, the Keilim are Tamei. 1
Even though they are not supporting the weight of the stone on which the Zav is sitting.
QUESTIONS ON RASHI
Rashi writes that our verse discusses carrying
Moshav Zekenim: Our verse teaches only about the last things mentioned, i.e. Mishkav and Moshav.