Measurement of parasitic y-caps
Note that you cannot measure any AC parameter in this circuit when you have 0 Ohm AC connections on all nodes.
This is impossible to measure in-circuit.  The ground connection must be cut.
The only method is to measure series resonant frequency (SRF)  that demands an accurate model to choose how to perform the test with added reference caps, measurement method and impedance with types of probes.
I might use a scope probe loop to a spectrum analyzer to sniff any form of noise around the battery cable.  Then add some repetitive noise to excite bursts of resonance that may or may not dampen a lot with similar film caps added to match expected stray capacitance.  Keep in mind a battery is measured in (tens of) thousands of Farads so there is a parasitic equivalent series inductance ESL that now resonates with the parasitic capacitance which is in series with that loop.
Using a comparative circuit to balance the results is known as &quot;bridge&quot; method. This is done by changing a known value with a separate but similar circuit and expecting the same response or subtracting for a null value with some differential method and analysis.
This is done in many ways for other circuits.
Any questions?