The following strategies are used for wh-extraction of the complement of P:
(a) complementizer ku introduces the relative clause, and a resumptive pronoun appears in the extraction site (Muysken 1977, Dijkhoff 1983 ACS); although that pronoun takes the form of the 3rd person singular pronoun e, it is not sensitive to number, and may appear with plural antecedents;
(b) complementizer ku introduces the relative clause, and a detransitivized form of the preposition appears, formed by construal of a + P or pa + P: den/aden 'in, inside', dilanti/padilanti 'in front (of)', etc. (Muysken 1977, Kouwenberg & Murray 1994: 52). This strategy is not available for instrumentals, as no detransitivized form exists for the instrumental P ku. I have included examples of other detransitivized Ps under Value 7, for completeness.
(c) less frequently, a relative pronoun appears, which allows for pied-piping; this is a more minor strategy than (a) (see Feature 92 "Subject relative clauses" for discussion of relative pronouns).
(d) also marginal is the strategy whereby complementizer ku introduces the relative clause, and a gap appears – a strategy which may represent the influence of the Dutch construction; unfortunately, I have no examples at hand.
The last two strategies may well represent the influence of contact with Dutch, which uses pied-piping fairly frequently, and which allows preposition stranding.
Source: nd: 27 June 2009, p.6
Source: nd: 20 June 2009, p.2
Source: nd: 20 June 2009, p.7
Source: Kouwenberg nd
Source: nd: 16 July 2009, p.9